As a background art of this technical field, there is PTL 1. This official gazette describes as follows. “A momentum dispersion function refers to a correlation function between momentum and position, and there is a correlation between momentum and position in a charged particle beam extracted from an accelerator system 2, and therefore, in order to ensure the treatment quality, it is important to eliminate this correlation when it is transported to the entrance of a gantry. The role of the beam transport system 3 is not only to transport a charged particle beam to a gantry 5, but also to eliminate the momentum dispersion function and perform the transport. In general, when momentum dispersion has occurred in the x direction in the accelerator, it is necessary to combine an x-direction bending magnet with a quadrupole magnet so as to cancel the dispersion in the x direction, and when momentum dispersion has occurred in the y direction in the extraction from the accelerator, it is necessary to combine a y-direction bending magnet with a quadrupole magnet so as to cancel the dispersion in the y direction. There are various systems in the extraction system from the accelerator, however, among them, some have a property in which time and momentum distribution have a strong correlation, that is, some have a tendency that the central momentum greatly changes with time. This is considered to be due to a cyclic variation in the magnetic field of the magnet in the above-mentioned accelerator or radiofrequency power, or the like, and as described in first to fifth embodiments, such a cyclic variation, that is, the strong correlation between time and momentum distribution can be eliminated by monitoring the cyclic variation linked with the operational cycle of the accelerator on a beam position monitor, and performing orbit correction so as to cancel a dynamic variation in the output of the beam position monitor”.
Further, this official gazette describes as follows. “In a case where there is a strong correlation between time and momentum in this manner, by combining a monitor with a dynamic steering magnet, the bending magnet can be omitted as compared with the conventional combination of a bending magnet with a quadrupole magnet, and a transport path having a momentum dispersion function of 0 can be realized by a small and inexpensive device”.